This invention relates to a polymerization process for preparing copolymers of two or more vinyl monomers.
Commonly assigned copending U.S. Application Ser. No. 08/954,880, filed Oct. 21, 1997 for Foaming-Resistant Hydrocarbon Oil Compositions by Grolitzer and Zhao describes polymeric acrylate compositions effective as foam suppressants at low concentrations in hydrocarbon oils over a broad range of temperatures encountered in use. Acrylate copolymers with chemical properties tailored for defoaming effectiveness at one temperature range are not effective at a significantly different temperature range. More particularly, if ethyl acrylate (EA) content or molecular weight of the acrylate copolymer is too high, defoaming at low temperature (about 25xc2x0 C.) is poor even though satisfactory at high temperature (up to about 100xc2x0 C.). Conflictingly, decreasing EA content and molecular weight to improve low temperature effectiveness sacrifices performance at high temperature. Physical blending of different acrylate copolymers to solve these contradictory requirements does not work and instead promotes foaming. To the best of applicants"" knowledge, foam suppressants before the invention of this and the copending application were designated for use over a narrow temperature range, typically requiring individually different compositions for effective use over different use temperature ranges.
It would be desirable to provide a process for preparing acrylate polymers with chemical properties effective in use over a broad temperature range.
Now improvements have been made which are capable of flexibly providing vinyl polymers which are effective over a broad range of use conditions.
Accordingly, a principal object of this invention is to provide a polymerization process for preparing vinyl copolymers which, during polymerization, produces a continuous gradual change in polymer composition and molecular weight.
Another object is to provide such a process which forms a polymer fraction with a certain monomeric composition during one phase and another polymer fraction of different monomer composition and molecular weight in a second phase.
A specific object is to provide such a process wherein the copolymer is acrylate copolymer useful in the foam suppressant application disclosed in the noted copending application.
Other objects of this invention will in part be obvious and will in part appear from the following description and claims.
These and other objects are accomplished by providing a staged method of forming vinyl, preferably acrylate, copolymer by solution polymerization which comprises: a) charging a positive amount up to 90% of at least two vinyl monomers of the vinyl copolymer to a first stage reaction zone; b) polymerizing the at least two vinyl monomers to about 10 to 90% conversion in the presence of an initiator at an initiator to monomers mole ratio of about 1xc3x9710xe2x88x924:1 to 5xc3x9710xe2x88x922:1, the monomer content of one of the at least two vinyl monomers of the copolymer formed in the first stage being higher than the content of another of the at least two vinyl monomers in the first stage polymer, the weight average molecular weight of the first stage copolymer being about 2000 to 500,000 Daltons; and c) continuing polymerization in the presence of an initiator in a second stage while continuously adding the balance of the monomers of the vinyl copolymer to the polymerization reaction mixture of the first stage at a monomers ratio lower in the one monomer than in the first stage monomers ratio, the initiator to monomers mole ratio in the second stage being different than in the first stage, the vinyl copolymer formed gradually continuously changing in weight average molecular weight from the first stage and decreasing in concentration of the one monomer during the second stage, the weight average molecular weight of the total copolymer of the first stage and the second stage being about 2,000 to 250,000 Daltons.